The demand for higher capacity optical communication systems pushed common design approaches of Wavelength-Division-Multiplexed (WDM) optical systems to their limits. A typical configuration of a point-to-point WDM system includes a number of optical transmitters, optical multiplexers, spans of transmission fiber, optical amplifiers (traditionally, erbium-doped fiber amplifiers (EDFAs)), dispersion compensating devices, optical demultiplexers, and a number of optical receivers. Unfortunately, the usable gain bandwidth for the traditional EDFA optical amplifiers is limited and narrow which limits the number of independent channels supportable in a system. In addition, signal distortion and noise growth do not allow for transmission over very long optical transmission links. These limitations led to alternate methods for amplification with lower noise and greater broadband capabilities that will allow more channels and for longer spacing in between amplification and thus longer transmission distances. Optical systems with such broadband capabilities are commonly referred to as ultra long haul, Dense Wavelength-Division-Multiplexed (DWDM) systems.
Raman amplification provides a solution for broadband amplification in modern ultra long haul DWDM optical systems. Stimulated Raman scattering, which is a well-known physical phenomenon, can be employed to build amplifiers to compensate for fiber loss in optical transmission systems. In particular, Raman amplification advantageously uses the fiber itself as the amplification medium. Specifically, the output from high-power (Raman) optical pumps lasers are launched into an optical fiber at a wavelength shorter (i.e., higher energy) than that of the signal(s) to be amplified. Amplification then occurs when the pump photons are converted by the stimulated Raman effect into new photons at the signal wavelength(s).
Use of the Raman effect to transfer energy from a set of pump lasers to a set of channels is a key element in current and next generation long haul optical systems. Because the process of Raman amplification is inherently non-linear, control of a Raman amplifier should take into account not only the target channel power, but also the distribution of input channel and pump powers. For example, the addition of a single channel can significantly affect the gain profile applied to the original channel set. Further, component aging and environmental effects can cause optical channel powers to deviate from their optimal channel power targets. For these reasons, a Raman amplifier is dynamically controlled.
The typical control of Raman amplifiers requires the adjustment of the power levels of Raman pump lasers to attain a target output spectrum (i.e., target gain profile). The power of the pump lasers is adjusted dynamically so that signal powers are as flat as possible relative to some given power target. The complicated, non-linear nature of the Raman process requires a measurement and feedback based control scheme. Existing control methodologies utilized in Raman amplifiers typically consist of an iterative process which initiates pump changes based upon an approximate Raman gain model. For each iteration of the control process, output channel powers are compared to a predefined target; when the channel powers match the target, the process terminates. The definition of an error function which quantifies this comparison provides the goal of the control methodology. Aside from reducing the deviation from target, the pump changes also need to satisfy several constraints on channel and pump powers (e.g., maximum power available from the pump, maximum power per channel, etc.).
For example, a linear programming method of determining optimal pump configurations may be utilized for broadband Raman amplifier control. In one embodiment employing this type of Raman amplifier control, disclosed by the teachings of U.S. Patent Application Publication 2004/0036954, published Feb. 26, 2004, and incorporated herein by reference in its entirety, a linear program based on a linearized model of Raman gain is created. The linear program can be solved using a standard linear program solver and the solution provides Raman pump settings to obtain an optimal gain profile.